1896-97-1 SPATIAL THRESHOLDS OF COLOl'RS. 225. 



SPATIAL THRESHOLDS OF COLOURS AND THEIR 

 DEPENDENCY ON CONTRAST. 



By VV. B. Lane, B.A. 



(Read 3rd April, i8gj.) 



If one takes any coloured object of smajl, definite size, say a piece of 

 pigment paper, and gradually either removes himself from it, or causes 

 it to be removed while he remains stationary, he will notice not merely 

 that \;he coloured object decreases in its apparent size as its distance 

 increases, but also that its colour becomes modified and even disappears 

 as the object becomes to the sight extremely small. Ordinary illustra- 

 tions of the same phenomenon are quite common in our daily 

 experience. We have all noticed, probably, how across a stretch of 

 two or three miles of water a small craft, say a skiff, painted green, 

 has appeared to have, at first, no colour whatever, being merely a 

 bright or a dark spot upon the blue surface of the water. But, on 

 gradual approach it has become invested with a colour quality which is 

 more than mere brightness or darkness, though it may appear at first in 

 a colour, such as ;t<'/, quite different from its true colour disclosed on still 

 closer approach. Such, among the many other examples which 

 probably suggest themselves in plenty, to anyone disposed to search his 

 past experiences, serve to show that in this fact there is scope for a 

 scientific inquiry to determine the minimum sizes at which the various 

 colours are seen coloured in their proper tone. This is the problem of 

 the Sjiace Threshold of colours in a general way. Of course, in reckon- 

 ing the sizes of the coloured objects we must consider their size as 

 relative to the eye observing them, for their absolute size does not 

 necessarily change when their size, as relative to the eye, may have 

 changed greatly. The absolute size, indeed, may change and the object 

 remain fixed in one place, which would equally, if the observer remained 

 stationary also, produce an alteration in the size of the object relative 

 to the observer's eye. We can, of course, obtain an exact expression 

 for the relative size of objects to the eye by means of the visual 

 angle which they subtend at the eye, which is the same whether 

 the object be multiplied in size or merel>- decreased in distance in the 



